Fluid-fluid explosive self-mixing, sometimes referred to as "steam" explosions in the particular circumstances where water is involved, is a common and well known hazard and phenomenon in industry, particularly the foundry industry. Such an explosion can occur, for example, when a hot molten metal falls into a bath of water or on damp earth. The violence of these explosions can be major. In the aluminum industry there have been accidents where more than 100 workmen have been killed and a whole foundry destroyed.
Such explosions are caused primarily by bringing a hot fluid-- e.g., hot molten metal, salt, or glass-- into sudden and close contact with a cold vaporizable fluid-- e.g., water, industrial solvents, or heat transfer fluids-- that have a high vapor pressure, say on the order of hundreds of atmospheres, when they are at the temperature of the hot fluid. Under these circumstances, an explosion frequently occurs if some kind of trigger pressure pulse forces the fluids into contact with one another. However, the explosion may not need to be triggered in all cases. In other instances, minor triggers-- e.g., delayed supercritical boiling, mechanical motion, and even bubbles of one fluid trapped by the other in the bottom of a container-- may cause the explosion. At any rate, once a rapid mixing begins, it is likely to continue until a fair fraction of the two fluids have exchanged almost all their heat and energy. Apparently, the mixing is self driven, and fluid instabilities allow one fluid to mix into the other in extremely small particles, as small as a micron in size, so that the heat exchange occurs in milliseconds or less time. The pressure of the explosion is limited by the vapor or " steam" pressure at the temperature of the hot fluid. may be 5,000 to 10,000 psi for molten metals and water.
A fluid-fluid self-mixing explosion is greatly feared in the situation where a nuclear fission reactor malfunctions, resulting in the melt-down of the reactor core. In the event of reactor malfunction, the neutron reactivity is shut down, but the fission product beta decays continue to emit heat at a rate of approximately 5% of fuel power. If the core is not cooled after the neutron reactivity is shut down, the core will melt.
Hence there are many back-up coolant systems, some of which are referred to as emergency cooling systems. If these should fail, then it would be possible for the core to melt into a pool of uranium oxide, fission products, fuel rod casing material-- i.e., zirconium-- and other stainless steel components. Upon melting of the tube sheet that supports the core, the molten mixture would fall into the remaining coolant which forms a residual coolant bath at the bottom of the reactor vessel, resulting in a fluid-fluid self-mixing explosion. It is quite possible that the explosion could be equivalent to roughly 1 ton of a normal high-explosive, a size sufficient to damage the reactor vessel or exterior safety containment structure.